When walking over flat terrain, humans achieve remarkable energetic efficiency by exploiting the passive mechanical forces inherent to the biomechanical structure of bipedal locomotion. The idea that energetic efficiency is achieved by exploiting passive forces in the body has gained traction in the field of biomechanics, but no research exists on the way these principles govern the control of walking when vision is needed to guide foot placement. We tested the hypothesis that when walking over rough terrain, vision is used to identify footholds that allow the walker to approximate the efficiency of walking over flat terrain. Subjects walked over an array of randomly distributed virtual obstacles that were projected onto the floor by an LCD projector while their movements were recorded using a full-body motion capture system. Walking behavior was analyzed during a full-vision control condition as well as in a number of other visibility conditions in which obstacles did not appear until they fell within a window of visibility centered on the moving observer. Analyses focused on the relationship between the size of the visibility window and the active (i.e., muscle-generated) forces needed to redirect the center of mass to avoid stepping on obstacles. When the visibility window restricted vision to less than two steps, additional energetically costly active muscular forces were needed to avoid obstacles. When the visibility window allowed vision beyond two steps, active forces dropped to baseline levels observed in the full-vision condition. The findings suggest that visual information from the immediate foreground is used to identify footholds that allow the walker to approximate the energetic efficiency realized when walking over uncluttered terrain. The theoretical framework provided by the dynamic walking approach to gait control suggests an elegant explanation of the functional significance of the two-step distance in the visual control of human walking.